1. Field of the Invention
The present invention relates to three-phase relay protection devices for electrical power supply networks, and more particularly a protection device by means of which a triple-pole tripping is achieved in the event that, after a single pole tripping has been executed, a new single phase fault occurs in any of the two remaining phases.
2. Prior Art
When a fault occurs in an electrical power supply network, an electrical transient wave propagates therein and, because of repeated reflections in various discontinuities in the network, such as transformers, stations and the sources of the fault, the currents and voltages contain transients which are disadvantageous for the measuring relays in the relay protection devices. The dominant frequencies in such transients depend on the distance to the fault and the network configuration. Investigations have shown that, even with the introduction of transient suppression filters, it is difficult to use, for example, conventional impedance relays for relay protection devices, the operating time of which is to be less than the time of a cycle. In case of higher system voltages, greater demands are placed on the rapidity of the relay protection devices, while at the same time the damping of the transient waves decreases. It is therefore more difficult to apply traditional measuring principles to the relay protection devices.
From, for example U.S. Pat. No. 3,956,671, it is known to utilize the direction of movement of the transient waves at a measuring point for determining the direction to the source of disturbance. By comparing the polarities of the current and voltage waves, for example, it is possible to determine whether the fault lies ahead or behind the measuring point.
In the case of single-phase faults in networks with high system voltages, particularly in radial or thinly meshed networks, it is sometimes desirable for the relay protection devices to release only the faulty phase (so-called single-pole tripping). Since the other two phases are intact, it is easier to maintain stability in the network, at least for some time until high speed reclosing of the originally faulty phase.
When single pole tripping has been carried out, a new single-phase fault may occur in one of the intact phases before the line is rapidly reclosed, which usually takes place within 0.5 to 1.5 seconds after the disconnection. In that case it is desirable to avoid a further single-phase tripping, since only one intact phase would then remain and could not on its own maintain stability in the network. Instead, it is desirable in such cases that both of the two remaining phases are released. In conventional distance protection devices this is achieved by some type of undervoltage criterion from the released phase and impedance measuring in the intact phases. The disadvantage with this method is that it will be difficult to achieve operating times below 0.5 to 1 cycle of the fundamental frequency.